The invention according to the present application relates to fluid dynamic pressure bearings, spindle motors equipped with such fluid dynamic pressure bearings, and storage disk drive devices utilizing the spindle motors. More particularly, the invention relates to a leakage preventing construction of fluid dynamic pressure bearings, where lubricating oil enclosed in the bearings does not scatter or leak due to the effects of external forces, such as various shocks or vibrations, when the spindle motor is either operating or not operating.
Recent technological improvements have significantly increased the demand for smaller, thinner, and lighter data memory devices with higher density memory capacity. Magnetic and optical disks are now commonly utilized as storage devices. These new memory devices create a significant need for a new technology optimizing the rotational speed and accuracy of spindle motors used to rotate such magnetic and optical disks.
To satisfy this demand, there has been a growing trend to substitute conventional ball bearing systems, previously used to support a shaft rotating a magnetic or optical disk, with fluid dynamic bearing systems utilizing a lubricant and/or air as its medium to generate fluid dynamic pressure supporting the rotating shaft.
Such fluid dynamic bearings are already widely known, and are also known to have been applied in bearings for spindle motors. (For example, see Japanese Patent Publication No. 2937833 and U.S. Pat. No. 5,667,309) An example of a conventionally known fluid dynamic bearing is shown in FIGS. 12-15.
Conventional fluid dynamic bearing 06, shown in FIG. 12, includes a shaft 011 rotating within a bearing sleeve 07. Shaft 011 is supported for rotation by fluid dynamic bearing components located inside bearing sleeve 07. Thrust plate 019 is affixed to one end of the rotating shaft. Bearing sleeve 07 includes an inner cavity formed to receive thrust plate 019. Counter plate 018 encloses the inner cavity of bearing sleeve 07 such that the counter plate and the thrust plate are positioned in an opposing relationship with respect to each other. A continuous bearing gap 021, 022, 023 is formed between rotating shaft 011 with thrust plate 019, on one hand, and bearing sleeve 07 with the counter plate 018, on the other. Lubricant 012 is contained in this continuous bearing gap.
A radial fluid dynamic pressure generating groove 024 is formed on the inner circumferential surface of bearing sleeve 07. A first set of thrust fluid dynamic pressure generating grooves 025 is typically formed at the ceiling of the inner cavity of bearing sleeve 07 such that grooves 025 oppose the top surface of thrust plate 019. A second set of thrust fluid dynamic pressure generating grooves 026 is formed at the top surface of counter plate 018 such that grooves 026 oppose the bottom surface of thrust plate 019.
In the described bearing system, when rotating shaft 011 begins to rotate, dynamic pressure generating grooves 024, 025 and 026 generate fluid dynamic pressure gradients in the radial and thrust directions. The resulting fluid dynamic pressure suspends shaft 011 within the bearing space shaped by the surrounding bearing sleeve 07 and counter plate 018. The rotating shaft is supported by a lubricant film formed within the bearing gap.
A common concern with respect to the described system is that lubricant 012 contained within the continuous gap 021-023 may leak out of the bearing, specifically through the top opening of bearing sleeve 07. Several factors can contribute to the upsurge of the lubricant fluid level within the continuous bearing gap. For example, the upsurge may be caused by the expansion-contraction of the lubricant itself due to temperature change; by changes in the capacity level of a constructional element of the bearing due to the thermal expansion-contraction; by the pumping action at the time of starting and halting the operation of the spindle motor; and by the centrifugal force and pressure generated while the motor is running. There is typically a very small possibility of the leakage during shaft's rotation because of the in-pumping mechanism (the action by which the lubricant is drawn back into the bearing). However, the current problem in the art is that lubricant 012 can easily flow out of the opening of bearing sleeve 07 when the spindle motor is not operational, i.e., shaft 011 does not rotate, and an external force such as a shock or a vibration is applied to the spindle motor.
Instances of shock or vibrations are particularly common in portable computers, which are often dropped while being carried. Spindle motors used in such portable machines are thinner, lighter and smaller than spindle motors utilized in non-portable models. The risk of experiencing shocks and vibrations is much higher for such spindle motors. Therefore, there is a significant need for construction of shock-proof and vibration-proof models preventing the lubricant leakage.
Seal structure shown in FIGS. 12-15 attempting to prevent leakage of lubricating oil from a conventional fluid dynamic pressure bearing is typically called a “taper seal structure.” In this conventional structure, taper surface 035 is provided along the inner surface of bearing sleeve 07 at the opening portion of the bearing gap (the opening portion of bearing sleeve 07). Taper surface 035 is inclined at a specific inclination angle α and thus gradually broadens the bearing gap towards the opening. Consequently, a gradually expanding gap portion 029 with a wide opening is formed at the top of the bearing gap. This gradually expanding gap portion 029 also serves as a reservoir of lubricating oil because lubricating oil which flows out of the bearing gap is received and contained within this reservoir by the surface tension. As shown in FIG. 14, the structure shown in FIGS. 12 and 13 can also be provided with a peripheral groove 036 formed inwardly to the taper surface 035 along the inner surface of bearing sleeve 07.
Similarly to the prior art bearing shown in FIG. 13, the above-mentioned Japanese Patent No. 2937833 discloses a gradually expanding modified gap portion with a wide opening formed within the bearing gap outside the radial bearing area, where an oil accumulating circumference groove (similar to the peripheral groove 036 of FIG. 13) is formed on the inner surface of the bearing sleeve. According to the disclosure of the '833 patent, the inclination angle α of the gradually expanding modified gap is set at 0° or greater. The fact that the angle of inclination α is set at 0° or greater indicates that a portion of bearing sleeve's inner wall within the modified gap region can be parallel to the rotating shaft.
As explained above, lubricating oil reservoir 029 of conventional fluid dynamic bearing 06 shown in FIGS. 12-15 has a general triangular shape and is formed as part of the expanding portion of the bearing gap. However, increasing the capacity of the reservoir having this configuration is limited to either increasing the angle of wall inclination α or increasing the length of the reservoir in the axial direction, i.e., increasing the depth of oil within the reservoir.
For smaller, thinner, and lighter spindle motors, the axial length of fluid dynamic bearings is shorter. If the axial depth of the modified expanded gap opening 029 is too large, the axial length of radial pressure generating area could be shorter than the length required to generate sufficient dynamic pressure to sustain the shaft. Therefore, restrictions are applied to the axial depth of the modified expanded gap opening 029 accumulating oil to prevent lubricant outflow. Specifically, the axial depth of the modified expanded gap opening 029 must stay within the range where it does not affect the axial length required for the pressure generating section of the bearing. The structure of thinner and lighter spindle motors also places restrictions on the inclination angle α of the bearing gap wall. Consequently, the above two considerations complicate the use of the modified expanded gap reservoir 029, in a smaller and thinner design of a fluid dynamic bearing requiring no lubricant leakage when a shock or vibration is applied to it.
Furthermore, in the shown conventional design of the lubricant outflow prevention structure, a modified expanded gap opening 029 is designed to reach a maximum size at the opening of the bearing sleeve 07, and there is no mechanism inhibiting the scattering outflow of the lubricant. Consequently, when the rotating shaft undergoes a shock or a vibration during its rest time, the lubricant inside the bearing gap of the fluid dynamic bearings cannot remain within the modified expanded gap opening 029 and splatters outside the bearing. Thus, the conventional structure allows the lubricant to scatter outside the modified expanded gap opening 029 and, consequently, outside of the bearing. (See FIG. 15.)
The lubricant leakage problem of the conventional spindle motor is a direct consequence of the fluid dynamic bearing design, where the capacity of a lubricant reservoir is limited due to a number of restricting factors and lubricant outflow can not be prevented. Additionally, in the conventional oil reservoir there is no prevention wall protecting the bearing against lubricant leakage when a shock or vibration is applied to the resting shaft. With the need to meet an increasing demand for smaller, thinner, and lighter spindle motors, the problem of making smaller and thinner, yet leak-proof, fluid dynamic bearings must be solved as soon as possible.
In addition to the above described conventional structure of a lubricant reservoir, Japanese Patent Publication No. 3431723 discloses a fluid dynamic bearing having a lubricant-filled gap between the sleeve and the rotating shaft and fluid dynamic pressure generating grooves, wherein a large diametric section is provided on the interior surface of the sleeve's opening and a circumferential groove is formed in the middle of the large diametric section of the sleeve. This structure attempts to control the outflow of lubricant oil and its scatter. However, the details of the structure of the circumferential groove and the large diametric part are not disclosed.